Introduction to Silicates and Rock Types

Silicon-oxygen tetrahedron

Each silicon-oxygen tetrahedron (SiO₄) carries a -4 charge.

Zeolites

Can be rehydrated when immersed, Use as desiccants (e.g., removal of H2O from gaseous hydrocarbons and petroleum) • Cation exchange properties - Water softener • Ca2+ in water (hard water) is replaced by Na2+ from zeolite (soft water)

Nesosilicates

No shared O, Si:O = 1:4.

Sorosilicates

1 shared O, Si:O = 2:7.

Cyclosilicates

2 shared O, Si:O = 1:3.

Inosilicates (Single Chain)

2 shared O, Si:O = 1:3.

Inosilicates (Double Chain)

2 or 3 shared O, Si:O = 4:11.

Phyllosilicates

3 shared O, Si:O = 2:5.

Tectosilicates

4 shared O, Si:O = 1:2.

Pyroxenes

Composed of single SiO₃ chains.

Enstatite

Mg-rich end-member of pyroxenes group. (found in Peridotite, gabbro, basalt) (found in mafic rocks)

Ferrosilite

Fe-rich end-member of pyroxenes group.

Augite

The most common pyroxene

Wollastonite

Found in metamorphosed Si-rich limestone. A unit repeat of 3 twisted tetrahedra. Manufacture of tile.

Rhodonite

Forms via metamorphic fluid infiltration. % twisted tetrahedra.

Amphiboles characteristics

Asbestiform - fire retardants, thermal insulators, chemically resistant material, high tensile strength, very flexible fibers.

Not all amphiboles are asbestos

Hornblende

The most common variety of amphibole that alters from pyroxenes to biotite or chlorite. Found in igneous and metamorphic rocks

Kaolinite

(TO) Dioctahedral, cation in octahedral sheets = Al.

Serpentine

(TO) Trioctahedral, cation in octahedral sheets = Mg. Alteration product of olivine, pyroxene, amphibole

Pyrophyllite

(TOT layers connected through weak van der Waals bonds) Dioctahedral, Al in octahedral sheets.

Talc

(TOT layers connected through weak van der Waals bonds) Trioctahedral, Mg in octahedral sheets. Alteration product of olivine, pyroxene, amphibole

Muscovite

Dioctahedral, large cation connecting TOT layers = (K+)

Biotite

Trioctahedral, large cation connecting TOT layers = (K+)

Quartz

SiO₂ framework with no other cations. (hexagonal)

Feldspar

One Si4+ substituted by Al3+ (formation of alkali feldspars)

Plagioclase feldspar

Na-Ca solid solution (Albite to Anorthite).

Albite

(Na-rich end-member)

Alkali feldspar

KAlSi₃O₈ polymorphs (Sanidine, Orthoclase, Microcline).

Igneous Rocks

Textures indicate cooling rates: Slow: Large crystals; Fast: Small crystals or glassy texture.

Bowen's Reaction Series

Crystallization sequence affects mineral stability.

Sedimentary Rocks

Textures and common types (e.g., sandstone, shale, limestone) are influenced by depositional environment.

Metamorphic Rocks

Agents: Heat, pressure, fluids; Textures: Foliated (e.g., schistosity, gneissic) and non-foliated (e.g., quartzite, marble).

Index minerals

Indicators of metamorphic grade, e.g., Garnet in medium-grade rocks.

Olivine Group

Found in Mg-rich rocks (e.g., basalt, peridotite).

Garnet Group

Found in metamorphic rocks, extensive solid solutions.

Gold

Found in hydrothermal veins or placers.

Platinum

Found in ultramafic rocks (e.g., Bushveld Complex).

Hypogene minerals

Form at depth. Primary minerals

Supergene minerals

Form near-surface. Secondary Minerals

Supergene zone

Includes oxidized (Fe oxides) and reduced (enriched sulfides) zones.

Tectosilicates

Quarts, Alkali feldspars, plagioclase

Inosilicates

pyroxenes, amphilboles

Phyllosilicates

clays, micas

Know how to calculate the charge

each silicon atom in a tetrahedron has a +4 charge, and each oxygen atom has a -2 charge; the overall charge of an isolated SiO4 tetrahedron is -4, and the charge decreases by -2 for each oxygen shared with another tetrahedron

the difference between Pyroxenes and Pyroxenoid

pyroxenes having a more regular, repeating pattern of tetrahedra zigzagging along the chain, while pyroxenoids have a more irregular, twisted chain pattern

Characteristics of feldspathoids

Chemically similar to Feldspars, but contain 1/3 less silica

- Found in silica-deficient rocks

Plutonic/ Intrusive

rocks form during crystallization at depth within Earth

Volcanic / Extrusive

rocks form during crystallization at or very near Earth's surface

Volatiles

dissolved gases in melt, including

Rate of cooling (igneous)

Slow rate = fewer larger crystals - Fast rate = many small crystals - Very fast rate forms glass

Dark silicates (ferromagnesian; mafic)

Olivine, pyroxene, amphibole, biotite mica, Lower % of SiO2, Greater amount of K feldspar, More sodic (Na-rich)

Light silicates (non-ferromagnesian; felsic)

Quartz, muscovite mica, feldspars, greater % of SiO2, Lower amount of K feldspar, More calcic (Ca-rich)

Which minerals can coexist (under equilibrium conditions) in an igneous rock?

Quartz and K-feldspar under equilibrium conditions

Which minerals are more stable, given their crystallization sequence?

(last crystalizing minerals) quartz, muscovite, and sodium-rich plagioclase

Important agents of metamorphism

Heat, pressure, fluids

RECRYSTALLIZATION

minerals grow and develop an interlocking texture

NEOMORPHISM

• New minerals stable under new high T and high P conditions • Some minerals are found only in metamorphic rocks

There are several types of foliated textures in metamorphic rock

Gneissic texture (Banding) occurs during high-grade metamorphism

Schistosity occurs when recrystallization produces minerals large enough to see with naked eye

Rock cleavage occurs when metamorphic rock may be split into thin slabs

Burial metamorphism

low-grade metamorphism that occurs in the lowest layers of very thick accumulations of sedimentary strata

Metamorphic Grade

(intensity of metamorphism)

Index minerals

Indicators of metamorphic grade, e.g., Garnet in medium-grade rocks​

Olivine group

Mg-rich igneous rocks, used for Highly refractory, Peridot gem variety, readily alter to serpentine minerals.

Garnet Group

•Abundant in metamorphic rocks •Accessory in some igneous rocks
•Detrital garnet in sedimentary rocks
•Extensive solid solution in: • A site for Pyralspite group • B site for Ugrandite group
Uses for garnet paper and gemstones.

Zircon Occurence

Abundant in igneous rocks • Granite, Granodiorite •Metamorphic rocks • Gneiss, Schist •Accessory in sedimentary rocks • Very stable

Al2SiO5 Group Polymorphs

Abundant in metamorphosed aluminous rocks
• Mica schists
•When they occur together:
• P-T conditions of rock's origin ~ 500°C and 3.5 Kb •Often alter to sericite (fine-grained white mica) Manufacture of spark plugs and high refractory porcelains
•Index minerals in defining metamorphic zones - Sillimanite zone - Kyanite zone

topaz

•Formed by fluorine-bearing fluids during the last stages of crystallization of siliceous igneous rocks •Associated with tourmaline, cassiterite, apatite, fluorite, beryl, mica, feldspar, quartz

Beryl

Usually found in - Granitic rocks, Be-rich pegmatites, Mica schists, (Aquamarine; Morganite; Emerald; Heliodor) lightweight metal similar to Al

Tourmaline

•Most common in granitic pegmatites •Accessory in most igneous and metamorphic rocks
Gem varieties of Tourmaline (Schorl; Dravite; Elbaite; Rubellite; Indicolite)

Spodumene

important pyroxene source of lithium a lightweight metal.

Clay mineral group

Found in sedimentary rocks, Plastic behavior when wet and will harden when dried/fired - Highly reactive surfaces - Swelling properties - Some clays absorb water,

Kaolinite

- forms by weathering/alteration of feldspars and muscovite

Mica Mineral Group

It can be primary (crystallization from magma) or secondary (weathering/alteration of feldspars). It May weather to clay minerals, Common in Li-bearing pegmatites

Lepidolite

Common in Li-bearing pegmatites, - Source of Li (lightest metal)

Biotite

Characteristic of igneous and metamorphic rocks, May alter to chlorite, clay minerals (vermiculite)

Chlorite

Chlorite - as a secondary mineral in igneous rocks, it represents an alteration product of pyroxenes, amphiboles, biotite ii. Chlorite - as a primary mineral, very common in metamorphic rocks (diagnostic mineral of greenschist facies)

Macrocrystalline Variety Quartz

Visible crystals or made of large, intergrown crystals

Macrocrystalline Variety Quartz

Visible crystals or made of large, intergrown crystals

Primary twinning

Symmetrical intergrowth of two or more crystals of same substance (due to nucleation errors during growth)

Exsolution

Separation of initially homogeneous solid solution into two or more distinct crystalline minerals may exhibit exsolution feldspar group

Platinum occurence

Bulk found in core

Difference between the two polymorphs of Carbon

Diamond - covalent
Graphite - van der waals

"carriers" of dimonds

Xenocysts (kimberlites, lamproites)